spurious -ENOSPC on XFS

[Dave Chinner]

On Tue, Jan 13, 2009 at 04:58:01PM +1100, Lachlan McIlroy wrote:
> Christoph Hellwig wrote:
>> On Mon, Jan 12, 2009 at 06:14:36AM -0500, Mikulas Patocka wrote:
>>> Hi
>>>
>>> I discovered a bug in XFS in delayed allocation.
>>>
>>> When you take a small partition (52MB in my case) and copy many small 
>>> files on it (source code) that barely fits there, you get -ENOSPC. 
>>> Then sync the partition, some free space pops up, click "retry" in MC 
>>> an the copy continues. They you get again -ENOSPC, you must sync, 
>>> click "retry" and go on. And so on few times until the source code 
>>> finally fits on the XFS partition.
>>>
>>> This misbehavior is apparently caused by delayed allocation, delayed  
>>> allocation does not exactly know how much space will be occupied by 
>>> data, so it makes some upper bound guess. Because free space count is 
>>> only a guess, not the actual data being consumed, XFS should not 
>>> return -ENOSPC on behalf of it. When the free space overflows, XFS 
>>> should sync itself, retry allocation and only return -ENOSPC if it 
>>> fails the second time, after the sync.
> This sounds like a problem with speculative allocation - delayed allocations
> beyond eof.  Even if we write a small file, say 4k, a 64k chunk of delayed
> allocation will be credited to the file. 

The second retry occurs without speculative EOF allocation. That's
what the BMAPI_SYNC flag does....

That being said, it can't truncate away pre-existing speculative
allocations on other files, which is why there is a global flush
and wait before the third retry.....

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com